Introduction: Fibrotic hypersensitivity pneumonitis (fHP) is an immune-mediated disorder that remains a clinical challenge, as the ideal management strategy isn't yet clarified.

Aims: Access the efficacy of Mycophenolate mofetil (MMF) therapy in fHP patients and to investigate potential factors associated with progression.

Methods: We performed a single center retrospective study, analyzing electronic clinical records of fHP patients diagnosed in multidisciplinary team context under MMF during 12 months of follow-up. Clinical, laboratorial, radiological and lung function data were collected. Progression under MMF was defined according with the following criteria: relative decline of FVC above 10%; relative decline of FVC above 5% associated to worsened symptoms or imaging features; worsened symptoms and imaging features. The data was analyzed using IBMSPSS Statistics v23.

Results: A cohort of 104 fHP patients was analyzed. An overall significant decline of FVC (2,23±0,56L to 2,11±0,53L; p=0,004), TLC (3,99±0,83L to 3,82±0,55L; p=0,047) and DLCO (3,07±0,85L to 2,57±0,84L; p=0,014), after 1 year of treatment was noticed. The progression criteria were met by 44 (46.3%) patients. Only ground glass on HRCT (p=0,036) was associated with non-progressive fHP under MMF. No factor related to MMF response on predictive exploratory analysis was found.

Conclusion: Almost half of the patients under MMF in this cohort expressed a progression behavior, with solely ground glass on HRCT showing an association with non-progressive fHP. A longer follow-up and a multicenter prospective study design are warranted to confirm these findings and identify potential predictors of progression.